The present invention relates to a novel fluorine-containing polybenzoxazole and its precursor, a fluorine-containing polyhydroxyamide.
In general, polybenzoxazole has numerous characteristics including high mechanical strength in terms of tensile strength, bending strength and so forth, and excellent thermal stability in terms of thermal decomposition temperature, thermal deformation temperature and so forth, and is positioned as a high-performance engineering plastic comparable to polyimide. Japanese Examined Patent Publication No. 42-19271 discloses its basic structure and production method.
When assuming the application of these resins to protective films and so forth used for semiconductor parts, it is important that they have a low coefficient of thermal expansion simultaneous to a low dielectric constant. Namely, as a result of having a low coefficient of thermal expansion, these resins can be made to have a coefficient of thermal expansion substantially equal to silicon that is widely used as a substrate in the field of semiconductors, thereby being able to avoid the problem of warping of the device on which the film is formed. In order to achieve this, it is required to make contrivances to lower the coefficient of thermal expansion to a value similar to that of silicon. In the case of polyimide, attempts have been made to inhibit thermal expansion as a result of efficiently inducing planar orientation of the imide ring by making the molecular chain straight and rigid.
Japanese Patent Laid-Open Publications No. 62-207332 and No. 3-290434 disclose fluorine-containing polybenzoxazoles for imparting improved water resistance and new functions such as optical and electrical properties.
However, in the case of only introducing hexafluoroisopropylidene group as is observed in the structure of the conventional fluorine-containing polybenzoxazole, it is difficult to impart a sufficiently low coefficient of thermal expansion, and it is also not possible to accommodate requirements for a lower dielectric constant.
It is also desirable for polybenzoxazole to have a low dielectric constant and low coefficient of thermal expansion in the same manner as polyimide. With respect to these points, since polybenzoxazole has a cyclical structure free of carbonyl groups in comparison with polyimide, it is expected to have less polarization and a lower dielectric constant. In the case of polybenzoxazole into which fluorine has been introduced in particular, the dielectric constant is considered to be much lower than polyimide due to the additional decrease in electron density.
It is an object of the present invention to provide a fluorine-containing polybenzoxazole that demonstrates low dielectric constant and low coefficient of thermal expansion while maintaining the mechanical strength, thermal stability and other properties.
According to the present invention, there is provided a fluorine-containing polybenzoxazole comprising a structural unit represented by the general formula (1): 
where X is a tetravalent organic group containing an aromatic ring, and two sets of N and O are each bonded to the ortho positions of said aromatic ring of X such that two five-membered rings are formed next to said aromatic ring of X.
According to the present invention, there is provided a fluorine-containing polyhydroxyamide, which is a precursor of the polybenzoxazole, represented by the general formula (2): 
where X is defined as above.